Basic WP Query

basic-wp-query.php

<?php

$args = array(
	'post_type' => array( 'post' ),
	'post_status' => array( 'publish' ),
	'posts_per_page' => 100, // don't use -1, pick something reasonable
	'no_found_rows' => true, // useful when pagination is not needed.
	'update_post_meta_cache' => false, // useful when post meta will not be utilized.
	'update_post_term_cache' => false, // useful when taxonomy terms will not be utilized.
	// 'ignore_sticky_posts' => true, // ignore sticky posts
	// 'fields' => 'ids', // only good if all you need is ids
	'meta_query' => array(                  //(array) - Custom field parameters (available with Version 3.1).
		array(
			'key' => '_thumbnail_id', // only grab posts with featured images
			'compare' => 'EXISTS',
		),
	),
);
$the_query = new WP_Query( $args );

// The Loop
if ( $the_query->have_posts() ) :
	while ( $the_query->have_posts() ) : $the_query->the_post();
		// Do Stuff
	endwhile;
endif;

// Reset Post Data
wp_reset_postdata();

A short template of an WP query (see Bill Erickson's example for all the arguments).